Using 2 Arduinos doesn't really solve the basic problem of using PWM as a current source. The first post already found one of the problems. PWM is either on or off and sometimes the analogRead() landed on an "off".
I expect there is a chip available to do this. The MAX31865 does exactly this job for temperature sensors. It would probably work for this role, if there isn't another more-specialised chip available.
There are better processor chips too. Many non-Arduino chips have differential analog inputs so the V- probe goes to an input pin and not ground.